1. Field of Invention
The present invention pertains to an optical device with variable numerical apertures. More specifically, the invention relates to an optical device that uses special optical properties of the liquid crystal device (LCD) and an external voltage to generate focal points of different numerical apertures (NAs).
2. Related Art
Due to higher compatibility requirements for compact discs (CDs) and digital versatile discs (DVDs) in commercial products, current DVD pick-up heads should be able to read old CDs besides DVDs. However, the optical pick-up head of the CD drive adopts an optical system with a numerical aperture (NA) of 0.45 and a wavelength of 780 nm, which is completely different from that of the DVD drive with the NA being 0.6 and the wavelength being 650 nm. Therefore, it is impossible to use the same set of objective lens and optical system to read optical discs having these two different specifications. To ensure the requirement of backward compatibility, devices using two switching objective lenses or one objective lens with dual focal points are proposed. The design of two switching objective lenses uses two objective lenses with different focal lengths and one of them is selected at a time according to the type of the optical disc (CD or DVD). The selection is achieved by using a driving device. This design, however, would increase the weight of the whole optical pick-up head system and the price of the product. Therefore, it becomes the trend to look for solutions in the designs of one objective with dual focal points.
There are already various inventions and designs of optical pick-up heads with dual focal points. For example, the invention disclosed in the U.S. Pat. No. 5,446,565 proposed by Matsushita Electric Industrial Co., Ltd. in 1995 is a compound objective lens which uses a holographic optical element (HOE) with an accompanying objective lens to obtain focal points corresponding to different NAs. The basic principle is that the HOE has a 0 order penetrating light and a +1 order diffractive light. When the 0 order light passes through the objective lens, it will be converged at the focal point with the original NA, whereas the +1 order diffractive light will be diverged as when passing through a divergent lens so that the original parallel light will first diverge by a certain angle and then get focused by the objective lens, thus lowering the NA. The above idea has been realized into commercial products; wherein they further inscribe a holographic pattern on the objective lens to form the so-called dual-focus objective lens. The advantage is in that one single objective lens has two different NAs, which can be used to read two different types of optical discs. Nonetheless, since both the 0 order and the +1 order light beams exist at the same time one of them would be wasted while using the other to read a specific disc. Thus, the energy usage efficiency is lowered. Also the yield is not high enough because the holographic pattern is inscribed on an aspherical objective lens.
There is another design similar to that of Matsushita Electric Industrial Co., Ltd., also using an HOE to form a compound objective lens. The difference is on that it is an application of two wavelengths whereby two corresponding NAs are obtained.
Furthermore, another method is to use a liquid crystal device (LCD) and a polarizer to control the diameter of the beam incident on the objective lens, thus controlling the NA value. Please refer to FIG. 1A. When the electrode 20 on an LCD 10 is open, the whole LCD 10 plays the role of a birefringent medium which rotates the polarization of the incident light by 90 degrees so that the light with the rotated polarization can wholly pass through a polarization beam splitter (PBS) 30 and reach an objective lens 40. At this moment, the NA is that of the objective lens 40. With reference to FIG. 1B, if the electrode 20 on the LCD 10 is closed, the portion around which being plated with the electrode 20 will generate an electrical field in the interior of the LCD 10 due to the imposed external voltage. This electrical field changes the orientation of the liquid crystal molecules so that that area of the LCD 10 changes to a medium with only one refraction index. Thus, the polarization of light within the outer ring does not change after passing through the LCD 10 and will be reflected by the PBS 30. Nevertheless, the light in the inner ring is still rotated by 90 degrees and can penetrate through the PBS 30. Therefore, only the central part of the beam will pass through the objective lens 40 and converge so as to effectively minimize the NA. In summary, the prior art uses the switching on electrodes on the LCD 10 to control and change between different NAs in order to read optical discs of different specifications. Yet this design will encounter slight spherical aberrations when the thickness of the disc varies.
In addition, there is another technique which uses an LCD in the optical pick-up head. This technique is not for the application of dual focal points in reading different optical discs. Instead, it makes use of the birefringent property that refraction indices of the liquid crystal will change in response to the variation of an external electrical field to modify the optical path difference of an incident light. The design of the electrode pattern on the LCD can control the distribution of the refraction index and thus change the wave front of the incident light, correcting the coma aberrations caused by a disc tilt. This technology is particularly designed for optical discs with high capacities, high densities and sensitive to the tilting problem.
It is for sure that the development of optical pick-up heads must be evolved toward the direction of being able to read optical discs with higher capacities and densities while at the same time considering the issue on compatibility with those in the old formats, i.e. the so-called backward compatibility. Therefore, DVD pick-up head designs must take into account the backward compatibility with CDs. It is expected that if the new generation of optical discs comes out, there must be similar concerns about backward compatibility with CDs and DVDs for the new optical pick-up heads too. When the storage density of discs gets higher, the NA of the objective lens must increase in order to minimize the light spot. However, increasing NA is likely to result in aberrations due to a small amount of tilt between the disc and the objective lens. A thinner substrate is thus needed to avoid coma aberrations. This will in turn cause spherical aberrations. Then it is necessary to solve the compatibility problem among discs of several different thickness, capacities and densities. The aforementioned state of art only considers the compatibility in two types of optical discs. So it is advantageous to invent a simple scheme to solve the compatibility issue for more than two types of discs.
In view of the foregoing, the present invention provides an optical objective lens having several different numerical apertures (NAs) so as to have distinctive focal points for different types optical discs.
Another object of the present invention is to solve the problems of optical aberration corrections present in optical discs with high capacities and densities.
According to the above-mentioned objects, the present invention provides an optical device having variable NAs, which uses the special properties of the liquid crystal device (LCD) with external transparent electrodes and a circuit control unit to control the refraction index of the liquid crystal (LC) medium by imposing an external voltage. This design can achieve the diffraction effect similar to that of a phase grating.
The above concept of the LC grating is further used to design a diffractive optical element and to combine with an objective lens. When the external voltage is absent, the LCD is completely transparent and there is no diffraction. At this moment, the light spot converged by the objective lens is the original focal point of the objective lens (i.e., the system has the original NA of the objective lens). When the external voltage is imposed, the LCD will be modulated into a diffractive optical element by the electrical field and the diffracted light is converged at another focal point (i.e., producing another NA). This new focal point is different from the original one and is suitable another type of optical discs. Therefore, the present invention can switch among focal points needed for different types of optical discs by controlling on and off of the external voltage. That is, the optical device of the present invention can have at least two focal points. If the electrodes of the LCD are further designed with many different patterns or having different external voltages, a variety of diffracted beams can be obtained through the switching and control on the external voltages, which diffracted beams are then converged by the objective lens into distinctive focal points suitable for optical discs of different specifications.
Furthermore, the LC grating can be generalized into the LC hologram, which then combines with an objective lens to form a compound lens. The LC hologram can be controlled by an external voltage so that the diffracted focal point will have small changes in response to the small changes in the hologram pattern. This further finely corrects the out-of-focus problem due to inhomogeneous thickness between the two data storage layers in the optical discs with high capacities and densities.